A) Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a vertically aligned liquid crystal display device with visual angle compensation.
B) Description of the Related Art
As visual angle compensation technologies for a vertically aligned liquid crystal display device, methods using visual angle compensation plates having various optical characteristics have been proposed as described in the following.
JP-B-HEI-7-69536 discloses a use of a visual angle compensation plate having three primary refractive indices nx, ny and nz, the refractive index nz being smaller than the other two primary refractive indices nx and ny, and the axis corresponding to the smallest primary refractive index nz being parallel to the surface normal direction of the compensation plate (having negative refractive index anisotropy). This visual angle compensation plate is disposed between a liquid crystal cell and at least one of polarizing plates crossed-Nicol disposed on both sides of the liquid crystal cell. Used as the visual angle compensation plate is either a negative uniaxial compensation plate: a so-called C plate in which in-plane refractive indices nx and ny are equal and the optical axis is parallel to a surface normal direction of the visual angle compensation plate, or a negative biaxial compensation plate: a so-called biaxial plate in which in-plane refractive indices nx and ny are not equal.
Japanese Patent No. 3330574 also disposes a biaxial plate similar to that described in JP-B-HEI-7-69536 inserted between a liquid crystal cell and at least one of polarizing plates crossed-Nicol disposed on both sides of the liquid crystal cell. Japanese Patent No. 3330574 discloses that it is preferable to dispose the biaxial plate in such a manner that an axis corresponding to an in-plane larger primary refractive index of the biaxial plate, i.e., a delay phase axis, is made generally parallel or perpendicular to an absorption axis of the polarizing plate in the display plane, and to set an in-plane retardation of the biaxial plate to 120 nm or smaller.
Japanese Patent No. 3027805 also discloses a visual angle compensation plate disposed between a liquid crystal cell and at least one of polarizing plates crossed-Nicol disposed on both sides of the liquid crystal cell. Japanese Patent No. 3027805 discloses that a uniaxial compensation plate having positive refractive index anisotropy and an in-plane optical axis (nz=ny<nx where nz is a refractive index in a surface normal direction, and nx and ny are in-plane two refractive indices), a so-called A plate, and a C-plate such as described above are used as a combination of the A plate disposed on the liquid crystal cell side and the C plate disposed on the polarizing plate side, and that it is preferable to set an in-plane retardation of the A plate to 120 nm or smaller.
JP-A-2000-19518 discloses that it is effective to use the biaxial plate disclosed in JP-B-HEI-7-69536 and Japanese Patent No. 3330574 and the visual angle compensation plate of stacked A and C plates disclosed in Japanese Patent No. 3027805.
Japanese Patents No. 3330574, No. 3027805 and JP-A-2000-19518 design the conditions of an optical compensation plate based on the visual angle compensation principle of a vertically aligned liquid crystal display device shown in JP-B-HEI-7-69536 so as to obtain more effective visual angle compensation.
JP-B-HEI-7-69536 does not limit a range of a product of a birefringence and a cell thickness, i.e., a retardation in a cell cross section in a thickness direction, of liquid crystal material of a vertically aligned liquid crystal device. Japanese Patents No. 3330574, No. 3027805 and JP-A-2000-19518 have studied a range of a retardation in a cell cross section in a thickness direction. As a retardation in a cell cross section in a thickness direction, Japanese Patents No. 3330574 and No. 3027805 (refer to paragraph [0037] in both cases) disclose that the retardation is preferably a minimum of 80 nm and a maximum of 400 nm, and JP-A-2000-19518 (refer to claim 13) discloses that the retardation is preferably a minimum of 300 nm and a maximum of 550 nm.
The preferable range of a retardation in a cell cross section in a thickness direction of a liquid crystal layer disclosed in Japanese Patents No. 3330574 and No. 3027805 and JP-A-2000-19518 is applied to an active matrix type liquid crystal display device, typically a thin film transistor (TFT) liquid crystal display (LCD) (i.e., a full-dot type LCD).
There is a vertically aligned liquid crystal display device which performs a segment display and is simple-matrix driven. A retardation of a liquid crystal layer in a cross section of such a vertically aligned liquid crystal display device is desired to be set larger than that of an active matrix type liquid crystal display device, from the viewpoint of good on/off operation. It is desired to provide visual angle compensation techniques particularly effective for a vertically aligned liquid crystal display device having a liquid crystal cell with a large retardation in a cross section in the thickness direction.
Japanese Patent No. 3330574 discloses (in paragraph [0044]) that it is preferable to set a retardation of a visual angle compensation plate in a cross section in a thickness direction approximately equal to that of the liquid crystal cell in a cross section in a thickness direction, and discloses (in paragraph [0049]) that it is preferable to set a pretilt angle as small as possible (to align liquid crystal molecules generally perpendicular to an alignment film surface).
Also in a vertically aligned liquid crystal display device of a segment display type driven by a simple matrix driving method, the retardations of a visual angle compensation plate and a liquid crystal cell in a cross section in a thickness direction are designed to be approximately equal, and the pretilt angle is designed to be as small as possible.
However, in general, if a pretilt angle is too small, uniformity of falling directions of liquid crystal molecules upon voltage application becomes bad, leaving a possibility of display irregularity. Further, the shape of each segment of a liquid crystal display device of a segment display type is more complicated than that of a dot matrix type so that an oblique electric field is likely to be formed in various directions, particularly near at an edge. Because of this, a danger of display irregularity becomes high in the segment display type. It is effective to make a pretilt angle large to some extent, specifically 1° or larger (an inclination from a liquid crystal cell surface normal direction is 1° or larger), to suppress display irregularity even if oblique electric fields are formed.
However, in general, as a pretilt angle is set larger, sharpness of the transmissivity—applied voltage characteristics of a liquid crystal cell becomes gentle near at a threshold voltage. FIG. 30(A) shows the transmissivity—applied voltage characteristics at pretilt angles of 0.5° and 1.5° as viewed at a visual angle of 0° (as viewed in front of the cell), and FIG. 30(B) shows the transmissivity—applied voltage characteristics as viewed at a visual angle tilted to 45°. The characteristics at the pretilt angle of 1.50 (an angle of 88.5° relative to a liquid crystal substrate surface) has dull sharpness near at a threshold voltage than the characteristics at the pretilt angle of 0.5° (an angle of 89.50 relative to a liquid crystal substrate surface). Sharpness becomes dull, as the visual angle is tilted more.
Dull sharpness does not pose any problem in static driving such as a thin film transistor, since an off-voltage is smaller than a voltage at which sharpness becomes dull. However, in simple matrix driving at a ½ duty or larger, an off-voltage is just at the position of dull sharpness so that a transmissivity of an off-voltage applied segment (off-segment) becomes high corresponding in amount to the dull sharpness.
In segment display in particular, a transmissivity of an off-segment becomes higher than that of a nearby background so that a phenomenon that an off-segment can be viewed, i.e., a crosstalk, occurs and a display quality is degraded. A crosstalk becomes more conspicuous at a larger duty ratio (1/8 duty, 1/16 duty or higher), and becomes more serious at an increased display capacity (the increased number of display segments).